Electronic devices such as integrated circuits and discrete power devices generally have metal regions externally accessible to facilitate electrical contact. Similarly, electrical interconnect structures such as printed circuit boards have regions at which electrical connection to a second electronic entity is formed. (Electronic devices and electrical interconnect structures as well as other structures having conducting pads for electrical connection are generically denominated for purposes of this disclosure electronic entities.) In such structures the interconnection regions are typically denominated pads and are generally formed from metals such as aluminum, copper, nickel, gold, palladium, silver, and tin. The thickness and composition of the metal layers are dependent upon 1) the intended application, 2) the method used to create the contact, and 3) the final interconnect structure. For example, lead frame packages use aluminum pads on integrated circuits and silver coated copper pads on the lead frame itself. Correspondingly, aluminum integrated circuit pad thicknesses are generally in the range 0.6 to 1.5 μm and silver package lead thicknesses in the range 4 to 10 μm. Substrate based packages used for wire bonded integrated circuits have copper substrate pads that are coated with electrolytic nickel/gold, electroless nickel/immersion gold (ENIG), or electroless nickel/electroless palladium/immersion gold (ENEPIG). Substrate based packages for flip chip integrated circuits have copper pads that are coated with immersion tin, solder, ENIG or ENEPIG. Similarly, the pad surface area for these different metallization layers is dependent upon the design and generally range from 50 μm2 to 1 mm2.
A variety of techniques relying on introduction of a metal to the pad, e.g. soldering, is employed to produce an electrically conducting region that adheres to the pad and to the entity being connected to the pad. Such interconnect formation requires an electrically conducting interface between the pad and the electronic structure that is mechanically sound and that typically has a resistivity (including the resistance of the interfaces) of no greater than 2 to 5μ ohms.
Extensive research has centered on forming suitable interconnects. As previously discussed, one commonly used interconnect resulting from such research involves the formation of a nickel region overlying the pad with subsequent formation of a noble metal such as gold, in turn, overlying the nickel. The gold is employed because it is not easily oxidized and thus provides excellent electrical conduction. However, typical pad materials such as copper diffuse rapidly through gold with subsequent production of oxides such as copper oxide at the surface to be soldered. Such oxides make soldering difficult and tend to degrade electrical conductivity at the interface. To avoid diffusion resulting in oxide formation, nickel, as discussed above, is employed as an intermediary barrier layer between the pad materials such as copper and the noble metal such as gold. The nickel itself is not acceptable without the overlying noble metal since without such overlying region, it forms oxides that preclude reliable and acceptable solder adhesion.
Electrolytic plating processes are commonly used in nickel/gold stacked layer structures in some applications such as wire bonded ball grid array packages. However, in other applications such as flip chip ball grid array packages electrolytic processes have been found undesirable for plating the nickel and gold regions overlying the pad. It is impractical to make the requisite electrical connection for plating to each flip chip pad without incurring excessive costs or unduly limiting the number of pads present. Additionally, even if such connection could be made, it is even more difficult to maintain a uniform current density over all pads and thus to uniformly and precisely control the plated metal thickness. Such non-uniformities and lack of thickness control either degrades the ultimate device properties due to phenomenon such as gold embrittlement of the solder joint or leads to additional costs associated with the necessity to increase gold thickness to ensure adequate joint formation. Accordingly electroless plating procedures are employed for forming both the nickel region on the pad and the overlying noble metal region. Electroless processes such as described in Electroless Nickel Plating, W. Reidel, ASM International, Metals Park, Ohio, 1988 provide an alternative to electroplating for metals including nickel, gold, silver, palladium, tin as well as other metals such as copper. Accordingly, it is possible to form such metals in thin continuous and uniform regions.
Despite the excellent characteristics of the ENIG and ENEPIG structures some problems have been persistent. In particular, at what appears to be random instances, subsequent soldering of the ENIG and ENEPIG structures leads to poor adhesion between the pad and the solder layer resulting in mechanical failure of the connection. This problem denominated black pad in the trade is extremely undesirable because the entire device is fabricated before the defect becomes apparent through mechanical failure of solder joints. Additionally, because failure comes after soldering and the concomitant barrier to pad inspection, it is extremely difficult to identify the root cause of the black pad defect. Because the appearance of black pad problems is unpredictable and because of the severe economic costs associated with this defect, a recognition of the cause and subsequent solution would be extremely desirable. However, despite extensive attempts such resolution has been an elusive goal.